1. Field of the Invention
This invention relates to a technique for machining a surface of a bore of an engine cylinder block so that the bore surface is formed with a number of fine recesses.
2. Description of the Prior Art
A surface of a bore provided in a cylinder block for an internal combustion engine is preferably formed to be smooth so that friction between a piston ring and the bore surface is reduced. On the other hand, the bore surface is preferably formed with a number of fine oil reservoirs for reserving engine oil on the bore surface so that an oil film is ensured between the piston ring and the bore surface.
Japanese Laid-Open Patent Publication No. 5-57597 discloses an efficient method of machining such a bore surface as described above. In the disclosed method, the bore surface is machined by a honing machine provided with a honing stick made from abrasive grains each having a predetermined roughness. As the result of honing, a large number of fine streaks are formed on the bore surface to be further formed into oil reservoirs. Although this method provides an efficient formation of the oil reservoirs, the formed oil reservoirs have a low oil reserving capability. More specifically, each of the streaks 3 formed by the honing has a bore surface side opening 3a which is wider than an inner part 3z, as typically shown in an enlarged view of FIG. 3. This wider opening 3a results in a problem that each oil reservoir has the low oil reserving capability.
In order that the oil reserving capability of the oil reservoirs may be improved, the bore surface side opening 3a of the oil reservoir 3 is desired to be rendered narrower than the inner part 3z. An opening width of the oil reservoir 3 is usually fine, ranging between 4 .mu.m and 5 .mu.m. A sealing performance between upper and lower chambers of a piston is reduced when the opening width of the oil reservoir 3 exceeds the range. Accordingly, it is difficult to render the bore surface side opening narrower than the inner part by ordinary machining methods. Under these circumstances, an electrochemical machining is employed to form oil reservoirs 3 as typically shown in an enlarged view of FIG. 1(C). A bore surface side opening of each oil reservoir formed by the electrochemical machining is narrower than the inner part and accordingly, each oil reservoir has an improved oil reserving capability.
However, the electrochemical machining requires a longer machining time. Furthermore, waste fluid resulting from the electrochemical machining needs to be treated or otherwise, disposed of. These drawbacks increase the manufacturing cost of the cylinder block.